Rubber or resin compositions based on synthetic rubbers such as polybutadiene or on various types of thermoplastic resins have hitherto been used as golf ball materials. However, a number of golf balls have been disclosed in which some member thereof is in a foamed, or expanded, form.
For example, U.S. Pat. No. 6,688,991 discloses a golf ball having a core which contains a highly neutralized resin material that is foamed in order to control the moment of inertia of the ball. However, the resin in this art has been foamed; given that resin generally has a lower resilience than rubber and that foaming such a resin material lowers the resilience even further, the distance traveled by the golf ball is greatly reduced.
Also, J.P. No. 3,958,833 discloses art wherein a two-layer golf ball core has a center core that is made of a rubber composition containing a blowing agent. However, in this art, blowing can be carried out only in a target range within a single layer of the core. In addition, the two-layer construction of the core increases the production costs.
U.S. Pat. No. 5,688,192 discloses a golf ball having a compressible gaseous material dispersed at the interior. Also, J.P. No. 5,166,056 discloses art that includes, within a core-forming rubber composition, thermally expandable microcapsules containing a large amount of gas. However, in these disclosures, the compressible gaseous material and the thermally expandable microcapsules sometimes collapse due to the pressure applied during rubber curing, or may not properly expand, presenting difficulties during manufacture.